Lipoprotein lipase (LPL) is the major enzyme involved in the hydrolysis of triglycerides of VLDL and chylomicrons. LPL can be isolated by heparin affinity chromatography from human post heparin plasma. ApoC-II is a well known activator of the enzymic activity of LPL. ApoH is a recently described protein (beta2-glycoprotein-I)constituent of triglyceride rich lipoproteins in human lymph and plasma. ApoH has been shown to increase the enzymic activity of LPL in the presence of apoC-II by 45 plus/minus 17 percent. ApoC-III decreased the apoH and apoC-III enhanced activity by LPL by 77 percent. These results provide evidence that this new apolipoprotein, apoH, modulates the enzymic activity of LPL in triglyceride metabolism. The ultimate proportions of apoH, apoC-II, and apoC-III in triglyceride rich lipoprotein particles may determine the ultimate rate of LPL catalyzed triglyceride hydrolysis.